Such magnet pumps are used, for example, to provide the pressure to hydraulically adjust a gate valve of a coolant pump which is driven via a pulley so as to thereby control volume flow.
In these pumps, an armature of the electromagnet and together therewith an axial piston comprising an axial through bore are moved up and down in a cylinder by alternately feeding current to the coil. The through bore is closed at its end facing the outlet by a non-return valve which is also arranged in the cylinder. The discharge movement is effected against another non-return valve which rests upon an outlet of the cylinder. The cylinder is filled when the valve is reset since its outlet is closed by the second non-return valve, and the first non-return valve is lifted off the axial piston due to the negative pressure produced in the cylinder by the return movement. The fluid is again moved out of the cylinder when current is again fed. Feeding of current and non-feeding of current or partial feeding of current to the coil of the electromagnet therefore results in intermittent pumping.
Such an electric fluid pump is described, for example, in EP 0 288 216 A1. To prevent an undesired braking of the piston and/or the armature by the axial movement of the armature and the positive and negative pressures thus produced at the opposite axial ends of the armature, the two spaces in front of and behind the armature are connected with each other via axially extending grooves or corresponding deepened portions of the guide or the armature so that a pressure compensation can take place.
Another magnet pump or oscillation pump is described in WO 2011/029577 A1. In this pump, the axial piston is not fixedly connected with the armature, but is merely pressed against the armature via a compression spring. The unit of piston and armature is therefore inexpensive to produce since an offset of the guides can be compensated.
These conventional magnet pumps have the disadvantage, however, of not providing a fail-safe function. For example, if used to adjust an adjusting ring of a coolant pump, this means that in the case of a coolant pump closed by the adjusting ring and failure of the magnet pump that the pressure in the chamber can only be reduced very slowly through leakages via the magnet pump, or that additional drain valves must be used. When using a hydraulically controllable mechanical coolant pump, overheating of the internal combustion engine with the corresponding subsequent damage may otherwise, for example, occur.